1. Field of the Invention
The present invention relates to gear cutting machines and a method of cutting gears to provide gears having the optimum characteristics of tooth angle, face, proper top and bottom lands and spacing to provide gears of increased strength and of reduced operational noise levels to provide a gear of optimal efficiency for the particular application and material from which the gear is constructed. More particularly, the invention pertains to a method of gear cutting and an electronic variable speed motor and control system for controlling the cutter to obtain and maintain the proper index, helical angle and the number of starts by monitoring and controlling the cutter to efficiently cut a variety of gears for various applications utilizing a variety of metals from which gears may be constructed.
2. Description of the Prior Art
The prior art of cutting gears utilizing shaper cutters and hobbing machines includes a variety of apparatus and methods designed to improve gear cutting in the production of gears. One method for increasing the accuracy of hobbing machines has been to increase the size of the machine bed, the rotary machine table and the hob cutter to increase the gear tolerances attainable in the hobbing operation. It is well known in the gear cutting art that close tolerances including proper tooth size, diametral pitch, clearance, depth and backlash must all be controlled to not only assure proper meshing of gears but also to assure prolonged service life. These problems are further compounded in the cutting of helical and spur gears and the production of worm gears in which a change gear has heretofore been employed in the prior art to approach the proper helix angle. In these applications, and in other applications, gears are sometimes cut in at least two operations so that after the production of a rough gear, the gear is then further machined in a second machine for cutting at closer tolerances to provide the proper pitch lead and tooth profile. Under these procedures, it is possible to cut too small a gear or not cut enough material away from the gear blank thereby resulting in excessive wear of components in the second closer tolerance machine and a waste of time, labor and materials.
It is understood by those skilled in the art that the operational reliability, service life and efficiency of equipment utilizing gears depends greatly upon the precision and the quality of workmanship expended in the production of gears. In addition, it is known that service life, operational efficiency and the noise level and environmental considerations in work places utilizing machines with transmissions and gears depend upon quality, precision, and tolerances of the gears produced. Consequently, the production of gears having tolerances approaching the calculated theoretical values provides a means for the reduction of noise levels from the machinery and for providing stronger equipment.
As recognized by those skilled in the art, gear cutting is a discontinuous process that subjects the cutting machine to shock resulting in vibrations and a non-uniform toolwear that has been accommodated in prior art hobbing machines by either always setting the machine up so that the center portion of the hob cutter is utilized or by intuitive knowledge of the workman as to the amount of the hob travel generally encountered in the material of the gear blank, size and the type of gear being cut. The intermittent machining process encountered in hobbing operations provides a shock load that may result in machine vibrations and an increased random tool wear further resulting in the production of gears that do not provide the degree of tolerance necessary for optimized life and machine efficiency. As a result, such gears operate by generating more noise and are subject to fatigue, cracking and breaking.
The prior art has recognized the problems of accuracy in cutting gears and has attempted to solve these problems by utilizing larger machines having components of closer tolerance so that the travel, speed and friction imposed on the various components are more perfectly controlled to result in the production of more perfect gears. In U.S. Pat. No. 2,962,166, a gear sorting device is disclosed for the production of gears. In U.S. Pat. No. 2,962,166, the automatic gaging and sorting of the output of the machine tool is provided to alleviate a combination of defects such as errors in axial separation of the hob and gear blank, worn hob, hob shift or other such type of adjustments. Further, it is pointed out that random variations may indicate a hobbing machine adjustment even though the hob is sharp and the axial separations are substantially correct. As a result, in U.S. Pat. No. 2,962,166, the adjustments to the hobbing machine are not made until a plurality of gears have exhibited the same defect or defects. It is, of course, fundamental that making changes in the hobbing machine after the production of more than one gear may as a result of trial and error result in future gears having greater tolerances and preferred configurations. It is, however, further apparent that gears already produced may either be wasted or require further operations to put them into a usable condition.
In U.S. Pat. No. 3,254,566, an electronically controlled fluid motor powered machine is disclosed wherein an electronic computer means responsive to the rotation of the hob or cutter provides selective control by which preselected values may be correlated electronically and transmitted to actuate a lead screw and work spindle in response. However, in U.S. Pat. No. 3,254,566, the calculations and functions of the machine provide computer calculations for the speeds and relationships of the various axes involved and thereafter presume hob rotation and efficiency with respect thereto. Furthermore, hob shift and hob cutter control, which is one of the most important aspects in the production of high quality gears, are not suitably controlled. Moreover, U.S. Pat. No. 3,254,566 employs a hydraulic motor to provide a variable speed to thereby eliminate the traditional change gears. As will be recognized by those having knowledge of hydraulic systems, time lags of a few seconds are inherent in the use of fluid activated and controlled systems. As a result the response time of such systems are measured in seconds with the fluid system searching to catch up with the computerized system. As a consequence the method of hobbing provided in U.S. Pat. No. 3,254,566 does not contemplate the relatively instantaneous positive correction of errors contemplated by the present invention. In the present invention, the computer correlates and improves upon the efficiency of the gear cutting machine components such as base, bearings, feed components and rotational components to improve the quality of produced gears. However, improper machine set-up, operation and wear of the hob cutter and hobbing machine results in imperfect gears notwithstanding the theoretical calculations. As a result, the production of quality gears in the prior art has relied primarily upon the intuition of the workman cutting the gear to produce a more uniform and optimized gear by taking into account cutter efficiency, hob shift, rotational speed and the metallurgical composition of the gear blank. The resolution of this problem in conjunction with mathematical formulas and computations reveal many of the problems involved with translating theoretical computations into gears having optimum qualities.